A major problem associated with electrical distribution cable is its tendency, over a period of time, to fail due to the progressive degradation of its insulation. The degradative processes involved in the failure of cables are correlated with two "treeing" phenomena. "Electrical treeing" is the product of numerous electrical discharges in the presence of strong electrical fields which eventually lead to the formation of chnnels within the insulation material. These channels resemble the trunk and branches of a tree in profile under microscopic observation, from which the descriptive terminology derives.
The second type of treeing, known as "water treeing," is observed when the insulation material is simultaneously exposed to moisture and an electric field. This mechanism is much more gradual than electrical treeing, requiring an extended period of time to cause the degree of damage that affects the insulation characteristics of the distribution cable. However, since water treeing occurs at considerably lower electrical fields than required for the formation of electrical trees, this phenomenon is thought to be a leading cause of reduced service life of cables.
It is known that water treeing can be arrested to a degree by the incorporation of an anti-tree additive (e.g., various organosilanes) directly into the insulation composition of the cable at time of manufacture. Unfortunately, this approach only addresses the issue of how to inhibit the formation of trees and the associated deterioration of insulation integrity. It will, however, be recognized that a vast network of underground cable is already in place wherein the cable either has not been treated according to one of these procedures or it has degenerated significantly despite such efforts and is therefore subject to premature failure.
As a partial answer to industry's desire to extend the useful life of existing underground cables, it has been found that certain tree retardants can be introduced into the cable's interior to partially restore the insulation performance. An example of such a cable reclamation technique is found in U.S. Pat. No. 4,372,988 to Bahder. This patent teaches a method for reclaiming electrical distribution cable which comprises: purging the cable with a desiccant gas; then supplying, in a continuous fashion, a liquid, such as polydimethylsiloxane fluid, to the interior of the cable. This disclosure suffers from the disadvantage that the fluid can exude or leak from the cable. This reference addresses the potential loss of fluid by providing reservoirs which can maintain a constant fluid level, further adding to the complexity of this system. A similar method was proposed by Vincent et al. in U.S. Pat. No. 4,766,011, wherein the tree retardant fluid was selected from a particular class of aromatic alkoxysilanes. Again, the tree retardant fluid was supplied to the interstices of the conductor of the cable. However, fluid of this prior art disclosure could be polymerized within the water tree voids in the insulation and therefore would either not leak out of the cable or would only exude therefrom at a low rate.
As a variation on the method of Vincent et al., U.S. Pat. No. 5,200,234 to Bertini, assigned to the assignee of the present invention, discloses a method for the restoration of in-service electrical cable, which is positioned within a surrounding conduit, comprising filling the space between the cable and the conduit with a homogeneous mixture of a silane antitreeing agent and a dielectric oil, said dielectric oil being completely miscible with said antitreeing agent and having a solubility in the polymeric insulation of the cable of less than 5 weight percent.
The cable reclamation method disclosed by Vincent et al. is effective but typically requires a long exposure time to obtain a fully treated cable. As a consequence, a contractor might find it economically equivalent, or even advantageous, to completely replace a cable once it has deteriorated rather than avail himself of this restorative method.